Edge termination structures are well known in the semiconductor art and serve to achieve high breakdown voltages in planar device structures. Generally, edge termination schemes attempt to reduce the electric field at the edge of a device where the electric field might otherwise cause premature device breakdown due to junction curvature effects. Simply stated, edge termination structures serve to reduce current leakage and increase breakdown voltage in planar semiconductor devices.
One well known edge termination scheme employs "floating" guard rings. "Floating" guard rings are typically diffused or implanted rings disposed at the edge of a die about the periphery of a semiconductor device. These rings are not electrically fixed at any given potential and therefore "float". When a device is in operation, the rings attain a potential somewhere between zero and the drain voltage causing a depletion region to form around the "floating" guard rings. If successive rings are used, each ring extending from the main junction of a device to the outer edge of a die will be at a higher potential than the preceding ring. These guard rings spread the depletion region between themselves to reduce the electric field at the device surface.
In employing these "floating" guard rings, the spacing, width and doping of the rings are critical since they will determine the potential of the rings. If these parameters are chosen correctly, the depletion region at the edge of the device will be shaped in a fashion that decreases the electric field. Ideally, the electric field will be reduced to such an extent that breakdown will not occur at the edge of the device, but will occur in the interior of the device at the limit of the planar junction. If the spacing, width and doping of the rings are not chosen correctly, the device will exhibit severe leakage characteristics.
Typically, edge termination schemes employing "floating" guard rings utilize extremely high numbers of rings. This generally requires substantial amounts of real estate. Often, extra implants, diffusions and/or annealing steps are required. Finally, because these rings "float" and are not electrically tied to anything, they are extremely sensitive to surface contamination or charge. Disturbances on the surface may easily influence the potential of the guard rings.
Another well known edge termination scheme utilizes a highly resistive film such as alpha silicon or SIPOS at the edge of the device between the outer edge of the die and ground at the main junction. The resistive film is generally formed on a dielectric layer such as the field oxide. A small leakage current through the resistive film causes the potential to vary linearly from the die edge to the main junction. The fixed potential of the film causes the depletion region in the underlying silicon to spread out at the die edge thus reducing the electric field and increasing breakdown voltage. Deficiencies in the resistive film scheme are that it is extremely difficult to maintain film integrity and high resistance due to humidity and bias. Further, the high resistivity of the film can cause a large RC time constant between the film and the capacitance of the field oxide on which the film is disposed. This may cause premature breakdown during high speed switching.
Another edge termination scheme utilizes diodes disposed on the field oxide between the outer edge of the device and the main junction. These diodes span the entire perimeter of the die. The principle with the use of polysilicon diodes is basically the same as that of the resistive film, except that the high impedance of the diodes is obtained via reverse bias PN junctions instead of high ohmic resistance as with the high resistance film. The spreading of the potential at the die edge is achieved by placing several back-to-back diodes in series. The breakdown voltage of the diode chain is made higher than the breakdown voltage of the underlying junctions so that the diodes never avalanche, but only act to spread the depletion region at the edge of the device.
Because the diodes span the entire perimeter of the die, they require large amounts of real estate. For large area die, the individual diodes themselves become very large. The large diode size causes leakage at high temperatures to be a substantial concern. Finally, high voltage diodes are often necessary which add additional mask steps to the fabrication of the device.
Accordingly, it is highly desirable to have an edge termination scheme capable of reducing or eliminating the problems encountered in utilizing the prior art edge termination schemes.